1. Technical Field
The present invention relates to a technique for managing a public key certificate attached to an electronic mail to be received.
2. Related Art
Public key encryption is widely in use in encryption of data to be transmitted through a network and a digital signature. Japanese Patent Laid-Open Publication No. Hei 10-20779 discloses a technique of smoothly decoding data encrypted using a past key after the key has been changed and a technique for smoothly verifying data digitally signed using the past key. The technique is targeted to changing the keys without inconvenience and allows verification of a digital signature using an old public key when an electronic mail having an old transmission time is received. Although this technique is convenient, when an electronic mail in which the time of the electronic mail is intentionally changed is transmitted, an old public key is used which is not desirable from the point of view of security. In addition, in this technique, when update to a new public key is performed, information on the electronic mail on which the update is based is not recorded, and therefore, if the public key is automatically updated based on a mail with a harmful intent and a problem occurs, it is not possible to review the past transaction to search for a cause of the problem.